sin(9pi/7) = -sin(2pi/7)
Stuck....
Step by step would be appreciated very much.
Thanks
If you draw the terminal angles $\displaystyle \frac{9\pi}{7}$ and $\displaystyle \frac{2\pi}{7}$ on a unit circle, you will see that they differ by $\displaystyle \pi$. The sine of the angle is the y-coordinate of that angle/point on the unit circle. You will see that one is the negative of the other.
In general, $\displaystyle \sin (x + \pi) = -\sin x$.